1. Technical Field
The present disclosure relates to catalysts suitable for use in fuel cells, and specifically to platinum (Pt) multi-walled carbon nanotube (MWCNT) catalysts.
2. Technical Background
A fuel cell (FC) is a device that converts energy of a chemical reaction into electrical energy (electrochemical device) without combustion. A fuel cell generally comprises an anode, cathode, electrolyte, backing layers, and flow fields/current collectors. There are generally five types of fuel cells, as defined by their electrolytes:
TypeElectrolyteTemperatureCommentsPhosphoricLiquid175-200° C. Stationary power,acid (PAFC)phosphoriccommerciallyacidavailableMoltenLiquid solution600-1200° C. Molten carbonatecarbonateof lithium,salts, high(MCFC)sodium and/orefficiencypotassiumcarbonatesSolid oxideSolid600-1800° C. Ceramic, high(SOFC)zirconiumpower, industrialoxide/ytrriaapplicationsAlkalineAqueous90-100° C.Potassium(AFC)electrolytehydroxidesolutionelectrolyte, NASA,very expensiveProtonSolid60-100° C.Ionomer membrane,exchangeorganichigh power density,membranepolymercan vary output(PEM)quickly, portable/auto applicationsDirectSolid60-100° C.PEM that usesMemanolorganicmethanol for fuel(DMFC)polymer
The current disclosure is directed to catalyst material that are suitable for use with proton exchange membrane (a.k.a. polymer electrolyte membrane) (PEM) fuel cells (a.k.a. solid polymer electrolyte (SPE) fuel cell, polymer electrolyte fuel cell, and solid polymer membrane (SPM) fuel cell).
Among the various types of fuel cells, proton exchange membrane fuel cells (PEMFC) have unique characteristics, such as relatively low operating temperatures, high power densities and efficiencies, as well as the ability to respond quickly to changing power demands. A polymer electrolyte membrane fuel cell (PEMFC) comprises a proton conductive polymer membrane electrolyte sandwiched between electrocatalytic layers.
In a PEM fuel cell, the oxidation and reduction reactions occurring are:2H2→4H++4e−oxidation half reaction+O2→2H2O reduction half reaction
This electrochemical process is a non-combustion process which does not generate airborne pollutants. Therefore, fuel cells are a clean, low emission, highly efficient source of energy. Fuel cells can have 2-3 times greater efficiency than internal combustion engines and can use abundant and/or renewable fuels. Fuel cells produce electricity, water, and heat using fuel and oxygen. When hydrogen is used as the fuel, the only emission from a PEM fuel cell is water.
Since the voltage of a typical fuel cell is small, a number of individual cells are usually stacked in series. In addition, the two half-reactions normally occur slowly at the low operating temperatures of the fuel cell, thus catalysts are used on one or both the anode and cathode to increase the rates of each half reaction. Kinetic performance of PEM fuel cells is limited primarily by the slow rate of the O2 reduction half reaction (cathode reaction) which is more than 100 times slower than the H2 oxidation half reaction (anode reaction). The O2 reduction half reaction is limited, in part, by mass transfer issues.
As fuel, such as hydrogen, flows into a fuel cell on the anode side, a catalyst facilitates the separation of the hydrogen gas fuel into electrons and protons (hydrogen ions). The hydrogen ions pass through the membrane and, again with the help of the catalyst, combine with an oxidant, such as oxygen, and electrons on the cathode side, producing water. The electrons, which cannot pass through the membrane, flow from the anode to the cathode through an external circuit containing a motor or other electrical load, which consumes the power generated by the cell.
A catalyst is used to induce the desired electrochemical reactions at the electrodes. The catalyst is often incorporated at the electrode/electrolyte interface by coating a slurry of the electrocatalyst particles to the electrolyte surface. When hydrogen or methanol fuel feed through the anode catalyst/electrolyte interface, electrochemical reaction occurs, generating protons and electrons. The electrically conductive anode is connected to an external circuit to carry electrons. The polymer electrolyte is typically a proton conductor, and protons generated at the anode catalyst migrate through the electrolyte to the cathode. At the cathode catalyst interface, the protons can combine with electrons and oxygen to generate water.
The catalyst is typically a particulate metal, such as platinum, and is dispersed on a high surface area electronically conductive support, such as, for example, carbon black. Platinum (Pt) has been the most effective noble metal catalyst to date because it is able to generate sufficiently high rates of O2 reduction at the relatively low operating temperatures of PEM fuel cells. Proton conductive materials, such as Nafion®, are often added to facilitate transfer of the protons from the catalyst to the membrane interface.
Nanosized catalyst particles have unique characteristics such as the high specific surface area and superior catalytic activity, thus exhibit higher performance even at lower catalyst loading. Effective utilization of catalyst is typically feasible only through a homogeneous distribution of the catalyst on a high surface area support material.
For wide-scale commercialization, the PEMFCs need to overcome several challenges including cost and durability. Platinum is the most effective electrocatalyst for the PEMFCs because it is sufficiently reactive in bonding hydrogen and oxygen intermediates facilitating the electrode processes to form the final product, but the high cost of Pt limits catalyst loadings per unit area (or unit power output). The durability of PEMFC systems is typically controlled by the stability of membrane electrode assembly (MEA). For the PEMFCs to be commercially viable, fuel cells should meet the US Department of Energy's (DOE) targeted lifespan of 50,000 and 5,500 hours for the stationary and automotive applications, respectively.
One of the strategies to lower the Pt catalyst requirement is to improve the electrochemically active surface area (ECSA) by synthesizing nano-size particles with homogenous distribution on the surface of the catalyst support materials. There are several different procedures described in the literature to synthesize the Pt nanoparticles, such as the polyol process, electrodeposition, sonochemical processes, gas reduction, and a solution reduction method; however, when the size of Pt catalyst particles is <3 nm, the observed degradation of the resulting MEA incorporating the catalyst is dramatically increased. All these methods have been successful in yielding Pt nanoparticles, but the methods have produced wide particle size ranges due to agglomeration or inefficient control of nuclei growth.
The performance and durability of Pt catalysts for use in a fuel cell can be evaluated by accelerated methods, such as cyclic voltammetry (CV). For example, Pt nanoparticles can partially dissolve in an electrode during use, due to acidic conditions. Over time, this dissolution results in the loss of the precious metal catalyst. The dissolution of Pt in an MEA can be evaluated by cycling the potential applied to the MEA between 0.1 and 1.2 V to measure the loss of Pt electrocatalyst. When the upper voltage limit is increased from 1 to 1.2 V, the amount of dissolution observed is significantly increased.
MWCNTs and their composites have gained wide-scale interest as catalyst support materials due to their unique properties such as high chemical and oxidative stability, extraordinary mechanical strength, good electronic conductivity, high surface area and relatively simple manufacturing process. In contrast to traditional carbon black supports, the highly inert surfaces of MWCNTs necessitate surface modification to enhance the attachment of Pt nanoparticles.
Thus, a need exists for durable, fuel cell catalysts that can provide improved fuel cell performance and reduced cost over conventional catalyst technologies. These needs and other needs are satisfied by the compositions and methods of the present invention.